High intensity alkali metal vapor lamps of the present kind are described in U.S. Pat. No. 3,248,590 -- Schmidt, entitled "High Pressure Sodium Vapor Lamp." These lamps utilize a slender tubular envelope of light-transmissive ceramic resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises an amalgam of sodium and mercury along with a rare gas to facilitate starting. The ends of the alumina tube are sealed by suitable closure members affording connection to thermionic electrodes which may comprise a refractory metal structure activated by electron emissive material. The ceramic arc tube is supported within an outer vitreous envelope or jacket generally provided at one end with the usual screw base. The electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact.
High pressure sodium vapor lamps are vacuum jacketed in order to conserve heat and maximize efficacy. The common practice since the commercial advent of the high pressure sodium vapor lamp has been to evacuate the outer envelope, flash a getter, suitably of barium or barium aluminum alloy, and seal it off. The getter is provided as a powder which is pressed into channeled rings which are flashed by coupling radio-frequency energy into them. Such getter rings are shown for instance in U.S. Pat. No. 3,384,798 -- Schmidt, of May 1968, "High Pressure Saturated Vapor Sodium Lamp Containing Mercury".